" The battery failing is the most likely culprit but you should check to see if there are any problems with either the notebook or Ac adapter before rushing out and replacing the battery.
I would start by checking the bios, start the system tapping F2 to enter the bios, you can also tap F12 and choose bios from the menu. Once in the bios look for the option for AC adapter warnings, if they are disabled enable them and save changes. If there is a problem with the adapter you should start getting errors about the adapter.
If the Adapter seems alright, disconnect the battery from the system and power down the notebook. Wait an hour or so and then replace the battery and restart the system and see if you continue to get the message. If you can run the notebook on battery only allow the battery to fully discharge and then charge it again and see if the problem returns." from here. First instinct is, bad battery. Hope this helps, good luck.